请教，关于212的Channel coding of control information中ack/nack编码的额外难题

hangl

212的5.2.2.6节中提到：

For the cases with FDD or TDD ACK/NAKmultiplexing when HARQ-ACK consists of one or two bits information, the bit sequence is obtained byconcatenation of multiple encoded HARQ-ACK blocks where is the total number ofcoded bits for all the encoded HARQ-ACK blocks. The last concatenation of the encoded HARQ-ACK block may be partial sothat the total bit sequence length is equal to.

这句话，concatenation of multiple encoded HARQ-ACK blocks 怎么理解？FDD不是最多反馈一个TTI的ack吗？怎么理解多个块的连接。